TC waterline

[Jeff]

Question about the TC waterline: Is it my imagination, or does the TC appear to be stern heavy?? I have looked at a number of C-dory pics and it is common to see the TC 255 lying low in her stern. Bob Austin noted that the space between the hulls is underwater, aft (wasn't able to resurrect the pic he had of the stern waterline). Is this an issue for either performance when underway or noise and comfort at anchor? I had a sailing trimaran for a time and the passage of seas between the hulls was fully open, fore to aft. Appreciate your thoughts on this since I am likely moving from a power sailor status to a power status in the next year or two. The TC appears to have a lot of advantages but this issue does raise some concern. Thanks for your feedback.
Jeff.

[thataway]

This is the waterline of the boat sitting static in fresh water--salt water might make the waterline up half an inch. The tunnel is closed at displacement speeds. I find that it will open at about 11 knots with the Permatrims installed. The water line foreward is about at the aft end of the pilot house.

I don't know how much effeiciency it costs at displacement speed, but it must be a little. I think that the boat will still handle about the same in 2 foot seas, if the tunnel is open or closed. There is some "noise" of water slapping under the hull--not sure how high the tunnel has to be to not have noise. But we have not "lost any sleep" over it.

A neighbor with a TC 24 says that his tunnel is open--lighter engines, lighter boat.

My personal feeling is that it is not that the boat is stern heavy, but that the hulls should have been made deeper. Also the tunnel should be rounded. I have discussed this with Malcolm Tenant, who is probably the number one cat designer and he feels that the tunnel should be open and should have no flat horizonatal surfaces under the boat. He also strongly favors semidisplacement cats over planing cats.

On the other hand, the boat does handle better at planning speeds cross seas and down seas, than other cats I have run. The interior is certainly one of the best for this size boat. The boat is more fuel effecient at fast speeds--but may not be in the lower speeds. It also will handle good size seas at the slow planing speeds.

All in all we are happy with the Tom Cat.

[drjohn71a]

Jeff,

Every hull configuration has some different results. While Bob of Thataway favors the more rounded tunnel shapes and the deeper hulls (or higher tunnel roof), those changes would alter the performance of the TC 255.

The closer you get to the rounded, deeper hulls, the more softness in control , similar to altering a single hulled craft from planing toward displacement contours.

If the hull/tunnel height was altered so that the tunnel completely cleared the water at low speeds, you would get more efficient hull speed performance, i.e., better miles per gallon at about 6 mph. That would be due to the reduction in wetted hull surface, and the less tectonic contours compared to the short legged, low tunnel TC 255 as it is now. You would also lose that HUGE forward bunk - almost 7X8 feet as it is now.

I think that the incredible feeling of stability at all hull speeds, from zero to maximum would be lost if one made these changes. Test drive a Glacier Bay - behaves more like a displacement vs. planing hull. Some advantages, some disadvantages - buy the one you like.

To my thoughts the TomCat's hull performance is in great part due to the low and squared tunnel. It gets pretty darned good mileage at 6-8 mph just as it is. The hulls seem to condense the waves and froth into an effervescent "pad" which supports the craft evenly upon planing with a faint buzzing, like riding on a million bumble bees. At a stop, as well as at all speeds, the lateral stability is remarkable. I, for one, would not want to change anything that could reduce these characteristics.

I guess the conversation is kind of like, "Why don't we make a Glacier Bay's hull more like a TomCat's hull?" in reverse. The family history of the TomCat is the Dory hull - flat, easy planing - and the TomCat is an extension of that. Planing hulls, semi-displacement and displacement hulls are different animals and are constantly being tweaked one way or the other in each new watercraft. Just buy the one whose performance matches your desires/needs.

Re: Low setting transom

The original TomCat 24 was designed for lower horsepower, smaller and lighter engines more than the recent high horsepower, bigger and heavier four cycle engines. One reason for the Armstrong brackets was to increase aft flotation. Loaded, sitting at the dock, I think the TomCat 255 tends to sit low aft. Full fuel tanks, generator, fish boxes and ice boxes in the cockpit area worsen this. It is just the nature of the beast.

The TomCat 255, however heavily loaded, planes easily at fairly low speeds and runs pretty flat and economically with level Permatrims.

Most local ppl who want to tour my TC 255 love how close the cockpit is to the water. Easy to fish and feels like you are almost right in the water.

While, I'd like to have a fully loaded boat that sits exactly level, there are few on the surrounding docks that will do so when heavily loaded. To reduce this, you could put less fuel in the tanks. How often do you need to go 300 miles without refueling? You could put more storage weight forward (right now I keep nothing in the underbunk or anchor side storage lockers).

I have not had anyone who operated my TC255 desire anything different in performance. Better to have a bit lighter bow in heavy seas too.

Test drive different cats. Evaluate them at rest, loaded and unloaded.
Just by carrying 100 gallons instead of 150 gallons, one can eliminate the low aft section at rest. Pretend you only have 100 fuel capacitY?

Good luck on your choice,

John

[Jeff]

Wow,
John and Bob, those are very worthwhile and thoughtful responses. Thank you for the time you took to answer my questions. You both have obviously invested in understanding this boat as well as owning it.
Jeff.

[thataway]

I have to disagree with Dr John about the hull figuration. The tunnel shape or height has nothing to do with the boat being a planing hull. The planing hull configuration is due to the flatness of the bottoms of the hulls.
Me thinks that he confuses the shape of the tunnel with the shape of the hulls. The rounded hull bottoms, such as the World Cat and Glacier Bay, are semi displacement hulls or displacement hulls--and not directly related to the concept of displacement hulls in mono hulls, since cats get their speed from the narrow beam of the hulls in relation to the length of the hulls.

There is more slam and slap in the flat topped tunnel. The greater effeciency would run from idle speed on up to a planing speed, not just at 6 mph. With the deeper hulls you would not loose the large bunk--there would be absolutely no change, but the boat would be 4 to 6" further off the water. The cockpit depth would remain the same. In fact this could be achieved by cutting the hulls and along the water line and adding 6" in depth--no change in the boat in any other way.

I think that the stability at rest, would be approximately the same--and the ride would be considerably better in steep seas. The maneuvering in a high speed turn would remain the same. The conversation is not to make a Tom Cat like a Glacier Bay--it is a way to decrease the slap and slam, and to make it more effecient at lower speeds. This has absolutely nothing to do with the hull forum (rounded bottom vs hard chine); I have no idea who designed the Tom Cat hull--but in the catmaran design world it is widely held that there should be only curved surfaces on the bottom of the bridge deck in the smaller cats--and this seems to hold up to the boats which are in the range of 70 feet or more.

Although the Armstrong bracket is immersed in the water at rest, it gives very little floatation, it does not extend fully to the bottom of the boat and had to be modified (shortened) to reduce the spray and not deep enough to compensate for moving the engines aft. Moving the engines, gave more cockpit and cabin space.

You have to get the hull up on top clear air thru the tunnel--and it is air thru the tunnel, compressed, which gives the good ride. I feel it is too bad that when the boat was scaled up from the 24 to the 255, and taking into account the heavier engines--which I agree on--that the builder didn't take the effort to change the mold to add a few inches, round the tunnel and make the hull ride better.

Regards,